This invention relates to an automated storage and retrieval system and more particularly to such a system including a vehicle position determining system.
Automated storage and retrieval systems of the type presently known in the art are generally rack structures which include a plurality of lanes and a number of aisles which extend orthogonally from each lane. Items being stored in the rack, such as loaded pallets, are positioned in the aisles. To retrieve a loaded pallet from a position within an aisle or to deposit a loaded pallet therein, it is necessary for a vehicle, commonly referred to as a transfer robot, to enter the aisle. Because transfer robots are expensive it has been found advantageous to have a single transfer robot service a number of the aisles which extend from a lane rather than to provide a robot for each aisle. To carry the single robot between the various aisles a vehicle commonly referred to as a transfer vehicle, which moves along the lane, is provided.
The use of such transfer vehicles and transfer robots has contributed greatly to the efficiency and economy of modern automated warehouses. Their use has, however, required the development of systems which are capable of apprising such robots and vehicles of their location within the storage system. Clearly, it is necessary that an unloaded transfer robot be aware of the fact that it is underneath the loaded pallet nearest the lane in an aisle so that the robot may retrieve such pallet and carry it to the transfer vehicle, which vehicle will then carry the loaded pallet and the transfer robot to a pick-up area. Similarly, it is necessary that the transfer robot, when carrying a loaded pallet, be aware of its arrival at a position proximate the pallet nearest the lane in an aisle so that the transfer robot may deposit its loaded pallet at that point. It is also necessary, of course, that the transfer vehicle, when carrying a transfer robot (with or without a loaded pallet), be aware of its position in a lane so that the transfer vehicle may stop and off-load the transfer robot at the proper aisle along the lane.
A number of different systems have been developed to provide the required position information to the transfer robot and to the transfer vehicle. For example, U.S. Pat. No. 3,973,685 discloses a system wherein transfer robots are provided with photo-electric sensors extending fore and aft thereof, so that the robots are apprised of their adjacency to the pallet in the aisle nearest to the junction of the aisle and the lane as the transfer robots arrive at such point of adjacency. Such photo-electric sensor systems, although operating satisfactorily in ideal environments, are frequently less than completely satisfactory in actual factory environment. This is due to the fact that substantial articles, such as, for example, pieces of lumber, frequently dangle or extend from the pallets and even portions of the load carried by the pallets themselves frequently dangle or extend therefrom. Such articles dangling or extending from the pallets form obstructions which, when struck by the sensors mounted on the transfer robots, frequently cause the misalignment of such sensor systems resulting in a decrease in the accuracy of the position information provided and even damage such systems to the extent that they become inoperative.
Numerous attempts have also been made to provide a system whereby the transfer vehicle is apprised of its position along a lane relative to each of the aisles extending therefrom. One such system incorporates a plurality of photo-electric sensors mounted on the transfer vehicle and a number of pieces of reflective adhesive tape adhered to stanchions or posts adjacent to each of the aisles. The pieces of tape are arranged in a pattern or code containing information identifying the aisle adjacent to which they are positioned. Such systems, although of value, are less than completely satisfactory for a number of reasons. For example, in a typical warehouse environment, the reflective tape quickly loses a substantial portion of its reflective capability due to grime coating the tape. Clearly, such a circumstance is intolerable in that a loss of reflectivity of a particular piece of the tape will cause the photo-electric system to "read" an incorrect position code. Another problem arises when it is desired to alter the positions of the various pieces of tape forming the code in that each piece of tape, once adhered to a surface, loses a substantial portion of its adhesive capability when removal and re-adhesion of the tape is attempted. Finally, the use of such reflective tape is less than completely satisfactory because the photo-electric sensing system frequently becomes slightly misaligned due to the vibration thereof which is caused by the movement of the transfer vehicle along an aisle. Inasmuch as the angle at which light is reflected from the reflective tape surface is equal to the angle of incidence, that is, the angle at which light is received by the tape, and misalignment of the photo-electric transmitter is magnified by the reflectivity of the tape. It is therefore clear that such a system will work less than completely satisfactorily in the event that light from the photo-electric cell is transmitted other than perpendicularly relative to the reflective surface of the tape.